1. Field of the Invention
This invention relates to a ball joint consisting of a housing having a cylindrical bore, a ball seat composed of a hard synthetic resin and fitted fixedly in the cylindrical bore, and a ball stud having a ball portion fitted fixedly in the ball seat.
2. Description of the Prior Art
In a conventional ball joint of this kind, a soft polyurethane resin is used to make a ball seat so that the ball seat can be inserted easily into a housing, as shown in Japanese Utility Model Publication No. 22205/1975.
The data which are obtained when a load is imparted to this conventional ball joint in the axial direction of the ball stud are as follows. A ball stud separating load is about 70 kg in the case where the diameter of the ball portion is 20 mm, and a seat extruding load which causes the ball seat to be separated from a socket in a direction opposite to the direction in which the ball stud is separated is about 230 kg.
Regarding a ball joint of this kind, it has been demanded that the strength thereof with respect to these loads be improved, and that the dimensions and weight thereof be reduced. In order to meet this demand, it is necessary that the material for the ball seat be changed from a soft polyurethene resin to a hard synthetic resin (a thermoplastic resin).
In this kind of ball joint, inserting and fixing a ball seat in a housing poses some problems. Namely, the elasticity of, for example, a hard thermoplastic resin of a ball seat to be inserted into a socket is formed is generally not higher than 1/10 of that of a soft polyurethane, so that such a ball seat is not deformed easily. Therefore, when a ball seat of conventional construction which is formed merely out of a hard thermoplastic resin is inserted into a socket, an annular flange provided at the closed side of the ball seat is broken, and the assembling of the ball joint becomes impossible.